1. Field of the Invention
This invention relates to an apparatus for optically detecting a property of a liquid. For example, this invention is applicable to an apparatus for detecting a density of methanol or a property of gasoline in a motor vehicle fuel.
2. Description of Related Art
In recent years, its has been proposed to mix an extraneous component such as methanol or ethanol with gasoline so as to realize a cleaner exhaust.
To detect a density of the extraneous component, optical liquid property detectors utilizing the refractive index of light have been proposed (for example, in Japanese Patent Unexamined Publication Nos. 62-216438, 62-232538, and 64-10152). Such optical detectors can be used to detect a volatility of gasoline to determine whether the gasoline is heavy or light.
This kind of liquid property detector is arranged to detect the light reflected in an interface between a prism and a liquid to be measured. However, a contaminative material is contained in the liquid, although its amount is small. If such a material attaches to the interface, the incident light is scattered, resulting a measurement error. For example, in case that the liquid to be measured is gasoline, high-boiling point materials (oxides, materials caused by deterioration) of gasoline, inorganic materials such as Mn, S, Zn, and Cu, and ester or glycolic materials from a sealing agent on a piping or other members attach to the interface. As a result, such contamination deviates a quantity of reflected light on a light receiving element from a true value. Accordingly, a centroidal position of the reflected light is changed to cause a detection error.